Our transcription: Local concentrations of the Earth's heat cause some parts of the mantle to expand, become less dense, and float upwards as far as the Earth's outer skin will allow. The warmer rock then spreads out and gradually radiates its heat through the overlying blanket of more brittle crust. As it cools, the rock contracts, once again becoming dense enough for gravity to pull it back down into the mantle. This constant churning of material within the Earth eventually buckles and breaks the crust producing earthquakes, volcanic activity, and majestic mountain ranges.